Deposition method

ABSTRACT

A method of depositing a hard metal alloy is described wherein a volatile halide of titanium is reduced off the surface of a substrate and then reacted with a volatile halide of boron, carbon or silicon to effect the deposition on a substrate of an intermediate compound of titanium in a liquid phase. The liquid compound on the substrate is then reacted in the presence of hydrogen to produce a hard deposit containing titanium and boron, carbon or silicon. Also described are products which may be produced by the above method.

This application is a continuation in part of application Ser. No.358,110 filed May 7, 1973, now abandoned.

This invention relates to the production of hard deposits on substrates.More particularly, the invention relates to the production of depositson substrates, as coatings, or the production of free standing objectsmade from a deposit after removal of a substrate. The deposits of theinvention have physical characteristics which are substantially improvedover those presently known to those skilled in the art.

The production of high hardness materials for wear or cutting purposeshas been approached in a variety of ways. High carbon steel has oftenbeen employed, frequently utilizing alloying ingredients such achromium, vanadium, tungsten, molybdenum, cobalt, and others to improvehardness, toughness and strength at various operating temperatures. Castcobalt alloys, such as "Stellites", and similar materials have also beenused for wear and cutting products. Another type of material has beencomposites of tungsten carbide or other carbides cemented with cobalt ornickel.

High carbon steel, with or without other alloying ingredients, hasexcellent bend strength, particularly at use temperature near roomtemperature, and quite high impact strength. High carbon steel, however,does not offer satisfactory hardness for wear resistant and cutting toolproducts, its hardness being about Vickers number 900 (Vickers hardnessnumbers are in kg/mm² and are designated in the Claims herein as VHN) ora Rockwell C hardness of about 65 to 70. Thus, high carbon and similartool steels have certain limits on their use.

Cast cobalt alloys, particularly those having high percentages of carbonand metal alloying species such as chromium, tungsten and others, havehardness values similar to those of high carbon steel. Moreover, theymaintain good hot hardness. However, these materials are more difficultto fabricate than high carbon steel, generally cost more, and are quitebrittle.

In order to overcome the physical and mechanical shortcomings of theaforementioned products and the difficulty in manufacturing them,attempts have been made to produce these materials by deposition. Highhardness materials are used as coatings on various types of substratesor are formed into free standing objects to produce wear parts or toolproducts. For example, commercially successful products having coatingsof titanium carbide over cemented tungsten carbide have been produced.Hardnesses of over 3000 Vickers with improved friction characteristicshave been achieved. By way of further example, some small diametertubing of tungsten carbide has been produced by deposition on a mandrelwhich is subsequently removed.

Deposits which have been produced commercially thus far, both forcoating substrates and for producing free standing objects, havesuffered certain drawbacks. Although hardness appears to be satisfactoryin some cases, the strength and toughness of the materials has oftenbeen lower than desired. Typically, such deposits have been produced bychemical vapor deposition techniques and have resulted in columnar grainstructures wherein the grain size is relatively large. Because of thegrain size and the columnar distribution of the grains, such depositshave tended to be relatively brittle and mechanically weak. Moreover,the production of hard metal coatings has generally required the use ofa relatively high substrate temperature and relatively low depositionrate during the chemical vapor deposition process.

It is an object of the present invention to provide an improved methodfor producing coated substrates and free standing hard metal products.

Another object of the invention is to provide coated substrates and freestanding hard metal products having improved physical characteristics.

Another object of the invention is to provide, on substrates, improveddeposits of superior quality.

It is another object of the invention to provide coated substrates withthe coating having extremely high hardness.

Other objects of the invention will become apparent to those skilled inthe art from the following description, taken in connection with theaccompanying illustrations wherein:

FIG. 1 is a schematic diagram of a chemical vapor deposition systemwhich may be employed in the practice of the method of the invention;

FIG. 2 is a photomicrograph at about 200 times magnification of a crosssection of a deposit produced in accordance with prior art chemicalvapor deposition techniques; and

FIG. 3 is a photomicrograph at about 200 times magnificationillustrating a cross section of a deposit produced in accordance withthe invention.

Very generally, the method of the invention comprises providing avolatile halide in a gaseous form of titanium. The volatile halide isthen reduced off the surface of the substrate to form a lower halide oftitanium. This lower halide is reacted in the presence of a volatilehalide of boron, carbon or silicon to cause deposition on the substrateof an intermediate compound of titanium which is in a liquid phase. Theliquid phase intermediate compound is then reacted on the substrate witha volatile halide of boron, carbon or silicon to produce a hard depositcontaining titanium and boron, carbon or silicon.

Chemical vapor deposition, or CVD, is a well known technique forproducing a coated substrate, In FIG. 1, one common type of CVDapparatus is illustrated which is used for coating a substrate 11, thelatter being shown as a generally cylindrical rod. The rod 11 issupported in a work holder or fixture 12 supported from a rod 13 restingon a disc-shaped base 14. The disc-shaped base 14 is supported on areactor base 15 which is provided with an annular groove 16 therein.

The reactor is completed by a heat proof cylindrical walled tube 17 ofquartz or similar material which seats in the annular groove 16 and issealed therein by an annular seal 18. The top of the quartz tube 17 isclosed by a rubber stopper 19 of conventional design removably securedtherein. There is, therefore, defined a reaction chamber 21 in which thedeposition process takes place.

In order to heat the substrate 11 to the desired temperature, as will beexplained, an induction heating coil 23 is provided surrounding theouter wall of the glass or quartz tube 17. The induction heating coil 23is supported by means not shown and is provided with leads 25 and 27 towhich the induction heating current is conducted from a suitable source,also not shown.

In order to regulate the pressure within the reaction chamber 21 andevacuate gasses therefrom, the lower wall or base 15 of the reactor isprovided with an opening 29 therein through which a tube 31 is passed.The tube 31 is suitably connected to a vacuum pump 33 and a vacuum gauge35 is connected in the line thereto for indicating the pressure withinthe chamber 21. By properly operating the vacuum pump 33, the pressurewithin the chamber 21 may be regulated as desired.

As gas inlet tube 37 is provided in the rubber stopper 19 through acentral opening 39 therein. There is optionally provided at the terminusof the tube 37 within the chamber 21, a porous basket 41 for purposessubsequently described. The tube 37 is connected through a plurality oftubes 43 and 45 to regulator valves 51 and 53 and flowmeters 59 and 61,respectively. Sources 67 and 69 of reactant gasses are connected to theflowmeters 59 and 61, respectively, for introducing some of the desiredreactant gasses for producing the chemical vapor deposition reactionswithin the chamber 21, as will be subsequently described.

A gas inlet tube 46 passes coaxially in the tube 37 and the porousbasket 41 to the region upstream of the substrate 11 in the reactor. Thetube 46 is connected by tubes 47 and 49 to regulator valves 55 and 57,and flowmeters 63 and 65, respectively. Sources 71 and 73 of reactantgasses are connected to the flowmeters 59 and 61 for introducingreactant gasses through the tube 46 to the chamber 21.

A known method in which a coating of high hardness is produced on asubstrate by chemical vapor deposition involves the introduction to thereaction chamber 21 of a volatile compound of the metal species desiredin the deposit. Typically, this is a metal halide. This material, ingaseous form, is passed over a heated substrate, on which heated surfaceit is decomposed to deposit the metal of interest. A gaseous reducingagent, such as hydrogen, may be mixed with the volatile compound of themetal to assist in reducing it on the heated surface of the substrate.Other gaseous compounds may be added to the gase stream, such as carbonbearing gasses, whereby compounds of the metal, such as carbides, areformed by chemical reaction on the heated surface. A more completeexplanation of the chemical vapor deposition process may be found inChapter 13 of the book "Vapor Deposition" edited by Powell, Oxley andBlocker, published by Wylie & Sons, 1966.

In FIG. 2, a cross sectional photomicrograph, magnified 200 times, showsa coated substrate produced by typical prior art CVD techniques, morespecifically set out in Example 3 below. The specimen was etched in amixture of dilute nitric and dilute hydrofluoric acid for about 30seconds at room temperature. It may be seen that the deposit iscomprised of relatively large columnar grains which are orientedperpendicularly of the substrate surface. Such deposits are typicallyquite brittle.

In each case involving the practice of chemical vapor deposition, effortis made to insure that the chemical reactions which cause the depositiontake place on the surface of the substrate. In other words, a reactionis caused which directly produces a solid deposit from the gaseousreactant or reactants on the surface of the substrate or mandrel.Heretofore, if the reaction was allowed to proceed in the gas steam awayfrom the heated surface, powdery non-adherent and non-coherent depositswere made.

The method of the present invention, although similar to chemical vapordeposition, is not truly that. The method of the invention employs adeposition apparatus essentially similar to a chemical vapor depositionapparatus, however, the apparatus is operated in such a manner that thetypical chemical vapor deposition process does not take place.

In accordance with the method of the invention, a sequence of events ismade to take place which is different from what has been believeddesirable by those skilled in the art. It has been discovered thatsuperior deposits can be produced by causing a chemical reaction off ofthe surface of the substrate resulting in an intermediate product whichis deposited on the substrate or mandrel in a liquid phase, and byfurther reacting the liquid phase on the substrate to form the desiredsolid phase.

A volatile halide of titanium is reduced off of the surface of thesubstrate to a lower halide. The substance for causing the reductionwhich may be introduced from the source 69 or may be provided inparticulate form as indicated at 75 is suspended within the porousbasket 41. In either case, there is occurring within the chamber 21 andspaced from the surface of the substrate 11, a reaction which produces alower halide of titanium. The chamber wall is kept at a sufficientlyhigh temperature to prevent condensation of the lower halide on thewall.

This lower halide is then reacted in the presence of a volatile halideof boron, carbon or silicon to cause the deposition on the substrate ofthe intermediate liquid phase. To cause this reaction, the volatilehalide of boron, carbon or silicon is introduced to the reactor at aregion which is downstream from the initial reduction reaction, such asthrough the tube 46 from the source 71. The further reaction produces afog or halo around the substrate which is observable during the processand which results in the deposition of a liquid on the substratesurface. This liquid is also observable.

The liquid phase which is deposited on the substrate is then reactedwith a volatile halide of boron, carbon or silicon to form the desiredsolid deposit. Although this mechanism is not entirely understood, it isbelieved that this reaction of the liquid phase, although slower thanthe reaction to form the deposited liquid, occurs relatively rapidly bycomparison with an all gaseous reaction, thereby contributing to higherefficiency and greater deposition rate. It has been determined that ifthe method of the invention is practiced, deposition can be effected atsubstantially higher rates at lower temperatures than with conventionalchemical vapor deposition.

Although not essential, the reactions above described are preferablycarried out in the presence of hydrogen gas. A flow of hydrogen in thereactor has typically resulted in much better process operation and morerapid deposition rates. Where hydrogen is used as the reducing agent inthe initial reduction of the titanium halide, the hydrogen is, ofcourse, already present in the reactor. Where titanium chips are used inthe bed 75, the hydrogen may be introduced through the tube 46 from thesource 73. Where hydrogen is used, it is preferred that the amount notexceed ten times the stoichiometric amount.

In practicing the method of the invention, the preliminary reductionpreferably occurs at a temperature not less than 700° C. The preliminaryreduction may be effected by passing the titanium halide through aparticulate matter such as titanium metal chips as peviously described,or by simply passing the halide with a reductant gas through a heatedzone, such as a heated bed of porous ceramic material or chips.

The method of the invention is capable of producing coatings whichexhibit extremely hard properties, e.g. in excess of 4000 Vickershardness number. These deposits are useful as coatings, and may be madeso thin as to produce a negligible change in the substrate dimension.From a commercial point of view, the coatings of principle interest aretitanium with boron.

These coatings may be applied to many different substrates such asgraphite; refractory ceramics, such as oxides; cemented tungstencarbides; refractory metal, such as tungsten, molybdenum, titanium, andeven iron, nickel or cobalt base materials. In the case of low expansioncoefficient materials the coating is regularly applied directly as anoverlayer, in other words, a buildup on the surface with no pretreatmentof the surface required. In the case of the iron, nickel and cobalt basematerials, wherein the expansion coefficient of the coating is vastlydifferent from that of the substrate, it is frequently necessary topretreat the substrate with a diffusion coating first.

A pretreatment where the difference in expansion coefficient is great,which has been investigated and proved to be particularly desirable, isthe diffusion of boron into the surface of the substrate. This diffusioncoating has the effect of increasing the compression strength of thesubstrate as well as changing the expansion coefficient near the surfacesomewhat. Although it has not been demonstrated experimentally, it islikely that other diffusion coatings into an iron, nickel or cobaltsubstrate would be useful before applying the hard coating. Otherdiffusion coatings which would perform a similar function would besilicon, carbon or nitrogen. The diffusion coating may be made by one oftwo well known chemical vapor deposition methods, pack-cementation or aflowing gas system. The latter is preferred for reasons which willbecome apparent during the description set forth below.

Boronization of an iron, nickel or cobalt substrate may be conducted atany temperature above 600° C. using a gaseous mixture of boron chlorideand hydrogen. The lower temperature limit is a practical one, dependentupon the acceptable rates of deposition and diffusion from a productionstandpoint. The upper temperature limit is that of the boron-metaleutectic formation. In the case of iron base alloy, the maximumtemperatures are in the order of 1150° C., for nickel base 950° C., andfor cobalt base about 1100° C. The precise temperatures depend upon theeutectic formation, which is influenced somewhat by the minor speciesinvolved in the alloys. However, maximum variation as a result of thepresence of such minor species seldom exceeds 50° C. Preferably, theboronization is to a depth of at least 10 microns, and shows up as clearwhite and a clear gray layer when etched in 2% nitric acid in alcohol.

As a practical matter, the preferred boronization temperature is about50° C. below the eutectic temperature to provide optimum productionrates. Optimum surface and dimensional fidelity of substrate material isobtained when boronizing is conducted no higher than 950° C. for ironbase, 850° C. for nickel base, or 900° C. for cobalt base alloys. Byhardening the substrate utilizing boronization, the hardening can beconducted at lower temperatures than any other method producingequivalent hardness, and amorphous or sooty deposits are avoided morereadily, thereby allowing the subsequent coating operation describedbelow. The boronized portion of the substrate appears as having a clearwhite layer and a clear gray layer when etched with 2% nitric acid inalcohol.

The boronized substrate surface, or other of the acceptable substratescited earlier, may then be coated with a thin deposit of titanium andboron, carbon or silicon. Best results have been achieved with titaniumand boron hard metal compositions. This codeposit may be maintainedextremely thin and provides an extreme degree of hardness for superiorwear and abrasion resistance in tool applications.

The method of the invention for the deposition of titanium and boron isas generally described above. A halide of titanium, such as titaniumtetrachloride, is flowed into the reactor and is reduced to a lowerchloride of titanium as an intermediate reaction. This lower chloride oftitanium flows over the heated substrate, is further reacted anddeposits with gaseous boron trichloride as liquid. Further reaction ofthe liquid occurs to form a solid deposit of titanium and boron. Theexistence of the liquid deposit has been demonstrated both by directobservation of the liquid formation on the surface and by inferentialdata. Methods of effecting the partial reduction of the titaniumtetrachloride are the flowing of the titanium tetrachloride through aheated bed of titanium chips, or flowing a mixture of hydrogen andtitanium tetrachloride over a heated surface of indifferent materialsuch as alumina beads. The resultant solid deposit, when the necessaryintermediate reaction is caused to happen, is either a smooth vitreousappearing coating or a botryoidal coating.

If the intermediate reaction is not caused to happen as, for example,directly injecting the titanium tetrachloride into the gas streamwithout the necessary preheat and high temperature reduction, a typicalcoarse hexagonal crystal of titanium boride is deposited. X-rayinvestigation demonstrates that either type of deposit is probablytitanium diboride. There is, however, a very substantial difference inthe properties of the two deposits as made by the two methods. Thedeposits made by the method of the invention, i.e. the vitreous orbotryoidal coatings, are extremely hard by comparison with those made bythe conventional chemical vapor deposition techniques. The harderdeposits regularly measure greater than 4000 Vickers and have, in fact,been measured at hardnesses of over 6000 Vickers. The variation is dueto the difficulty in the precise measurements of the thin coatings ofsuch hard materials. By comparison, a typical crystalline, orconventional titanium diboride coating has a hardness Vickers number ofbetween 2800 and 3200. This latter hardness is the hardness generallyaccepted in the trade for titanium diboride.

The following examples serve to assist in an understanding of theinvention:

EXAMPLE 1

High-speed steel 3.1 mm diameter drills were first boronized by passingan 8:1 volume ratio mixture of hydrogen and boron trichloride over themat a temperature of 950° C. at a pressure of 200 Torr for 15 minutes.The drills were then racked in a furnace, heated to a temperature of750° C. and maintained at a pressure of 200 Torr. Titanium tetrachlorideat a flow rate of 100 ml/min was passed through a bed of titanium chipsat the same pressure, heated to 850° C. Boron trichloride at a flow of400 ml/min and hydrogen at a flow of 800 ml/min were mixed with theeffluent from the chip bed and passed into the reactor furnace withoutcooling. In 40 minutes, a smooth, bright coating of 0.025 mm thicknessadherent to the steel was produced. After coating, the parts were heatedto 1150° C. and rapidly quenched in hydrogen gas to assure the hardnessof the steel at Rockwell - C 65. The coating had a hardness of 7000kg/mm² when measured with a 500 gram weight on a Vickers hardnesstester. The drills successfully produced 9000 holes in laminated glassfiber printed circuit board material as compared with 30 holes beforefailure for similar drills uncoated. Metallographic sections of thedeposit showed a lamellar deposit as shown in FIG. 3.

EXAMPLE 2

A cemented carbide rod of 1.5 mm diameter was coated in a manner similarto Example 1. No preliminary boronization was conducted. The titaniumtetrachloride at 100 ml/min and the hydrogen at 800 ml/min were passedthrough a bed of alumina beads heated to 700° C. before mixing with theboron trichloride at 400 ml/min. The gas mixture was directly injectedinto the furnace in which the drill rod was mounted with the furnaceheld at 850° C. A coating of 0.020 mm was made in 20 minutes. Thecoating was bright, smooth and adherent and had a Vickers hardnessnumber of 5800 kg/mm² measured with a 500 gram weight. Themetallographic section was similar to that of FIG. 3, showing the samelamellar structure rather than the well defined crystal structure ofchemically vapor deposited materials.

EXAMPLE 3

The experiment of Example 2 was run again except that the titaniumtetrachloride, hydrogen, and boron trichloride were directly injectedinto the furnace without any provision for preliminary reduction of thetitanium tetrachloride. The surface of the carbide drill rod wasslightly discolored but there was no measurable hardness increase.Metallographic examination showed only a slight coarsening of the grainboundaries near the surface and no well defined coating.

EXAMPLE 4

An experiment was run using direct injection of titanium tetrachlorideat 100 ml/min, boron trichloride at 400 ml/min, and hydrogen at 1600ml/min into the reactor furnace. The specimens were 1.5 mm cementedcarbide rods. The furnace was held at 1100° C. and the gases pumped offto maintain 200 Torr. After 60 minutes a bright coating of 0.008 mm wasachieved. The coating had well developed columnar hexagonal crystalswith a hardness of 2900 VHN.

The above Examples 3 and 4 illustrate the necessity for the reaction ofthe titanium tetrachloride to form a lower chloride which is depositedas a liquid in accordance with the method of the invention. In Example3, since there was no lower chloride formation and no possibility of theliquid deposition, no deposit was effected at the low depositiontemperature. In Example 4, the temperature of deposition was too high toallow liquid deposition so that the mechanism for the deposit was one ofordinary CVD and the columnar crystals were in evidence.

EXAMPLE 5

The process of Example 1 was repeated using 3.1 mm diameter high speedsteel drills. All conditions were the same except that carbontetrachloride at a flow of 400 ml/min was used instead of borontrichloride. The coating had a hardness of 4500 kg/mm² when measuredwith a 500 gram weight on a Vickers hardness tester. Drills did not failin 1000 hole tests on laminated glass fiber printer circuit boardmaterial.

EXAMPLE 6

The process of Example 2 was repeated using silicon tetrachlorideinstead of boron trichloride. The coating was bright, smooth, andadherent with a Vickers hardness number of 1650 kg/mm² when measuredwith a 500 gram weight. The metallographic section showed the lamellarstructure, similar to that of FIG. 3.

In performing the above Examples 1, 2, 5 and 6, the following reactionsare believed to be representative of the deposition mechanism:

    3TlCl.sub.4 (g) + Ti(s)→ 4TiCl.sub.3 (g)

or

    2TiCl.sub.4 (g) + H.sub.2 (g) → 2TiCl.sub.3 (g) + HCl

in the gas stream plus

    TiCl.sub.3 (g) + 1/2 H.sub.2 → TiCl.sub.2 (1) + HCl

resulting in deposit of a liquid on the surface followed by

    TiCl.sub.2 (1) + 2BCl.sub.3 (g) + 4H.sub.2 (g) → TiB.sub.2 + 8HCl.

EXAMPLE 7

An experiment was run using direct injection of titanium tetrachlorideat 100 ml/min; 50 ml/min silicon tetrachloride; 4200 ml/min hydrogen;and 7000 ml/min or argon to approximate conventional chemical vapordeposition techniques. The resultant deposit at 950 to 1000° C. was finecrystalline in superficial appearance and had a Vickers hardness numberof 950 kg/mm² when measured with a 500 gram weight. The cross sectionshowed typical columnar grains similar to FIG. 2.

Examples 6 and 7 show the difference between the method of theinvention, wherein a layered deposit essentially free of columnar grainsis formed from the intermediate liquid layer, and the conventionalchemical vapor deposition method.

Of particular significance with regard to this method is the temperatureat which the deposits can be produced at an acceptable commercial rate.It is well known that titanium carbide deposits as made by chemicalvapor deposition are regularly conducted in the range of 900° C. to1200° C. To produce a useful coating, in the order of 10 microns inthickness, several hours of processing are required. The method of theinvention provides a superior processing technique from this point ofview. The coatings which have superior properties, as noted may bedeposited at temperatures as low as 750° C. at a high deposition rate.Temperatures from about 650° C. to 950° C. are usually satisfactory.Processing times for depositing with the intermediate liquid phase areas little as 15 minutes for a 10 micron coating.

It may therefore be seen that the invention provides an improved methodfor producing a coated substrate, as well as improved quality coatedsubstrates. By providing an intermediate liquid phase on the surface ofthe substrate being coated, and subsequently reacting the liquid toproduce the final solid coating composition, the structure of thecoating composition is such as to provide superior physical qualities.More particularly, titanium-boron deposits of extremely high hardnessvalues result, as well as excellent resistance to corrosion. Othertitanium metal systems involving carbon or silicon can also beeffectively improved in their deposit quality in accordance with theinvention.

Various modifications of the invention in addition to those shown anddescribed herein will become apparent to those skilled in the art fromthe foregoing description and accompanying drawings. Such modificationsare intended to fall within the scope of the appended claims.

What is claimed is:
 1. A method for producing a hard deposit on asubstrate, comprising, providing a gaseous volatile halide of titanium,reducing said volatile halide off the surface of the substrate to form alower halide of titanium, reacting said lower halide in the presence ofa volatile halide of boron, carbon or silicon downstream from saidhalide reduction while maintaining the pressure and substratetemperature such as to cause the deposition on the substrate of anintermediate compound of titanium which is in a liquid phase, andreacting the liquid phase intermediate compound on the surface of thesubstrate with a volatile halide of boron, carbon or silicon to producea hard deposit containing titanium and boron, carbon or silicon.
 2. Amethod according to claim 1 wherein the lower halide is reacted in thepresence of hydrogen, and wherein the liquid phase intermediate compoundis reacted in the presence of hydrogen.
 3. A method according to claim 1wherein the substrate is metallic, and wherein the outer surface of thesubstrate is first reacted with a halide of boron, carbon or silicon ina gaseous form in the presence of hydrogen to form a diffusion layercontaining boron, carbon, or silicon at the outer surface of thesubstrate.
 4. A method according to claim 1 wherein the lower halide isreacted with a volatile halide of boron.
 5. A method according to claim1 wherein the lower halide is reacted with a volatile halide of carbon.6. A method according to claim 1 wherein the lower halide is reactedwith a volatile halide of silicon.
 7. A method for producing a harddeposit on a substrate, comprising, placing the substrate in a chemicalvapor deposition reactor and heating the substrate to a temperature ofbetween about 650° C. and about 950° C., providing a flow in the reactorof a gaseous volatile halide of titanium, reacting the volatile halidewith a reducing agent to produce a lower halide, providing a flowdownstream of the reduction in the reactor of a volatile halide ofboron, carbon or silicon, and controlling the substrate temperature, thereactor pressure and the relative amounts of titanium, and boron, carbonor silicon to cause the deposition on the substrate of a compound oftitanium which is in a liquid phase and a subsequent conversion of saidliquid phase on the surface of the substrate to a hard depositcontaining titanium and boron, carbon or silicon.
 8. A method accordingto claim 7 wherein a flow of hydrogen is provided in the reactor.
 9. Amethod according to claim 8 wherein the amount of hydrogen in thereactor does not exceed ten times stoichiometric amounts.
 10. A methodaccording to claim 8 wherein the titanium halide is reduced by passing amixture of hydrogen and the titanium halide through a heated zone heldat not less than 700° C.
 11. A method according to claim 8 wherein thevolatile halide of titanium comprises titanium chloride, and wherein thevolatile halide of boron, carbon or silicon comprises a chloride.
 12. Amethod according to claim 11 wherein said last-named halide comprisesboron trichloride.
 13. A method according to claim 11 wherein saidlast-named chloride comprises carbon tetrachloride.
 14. A methodaccording to claim 11 wherein said last-named chloride comprises silicontetrachloride.
 15. A method according to claim 7 wherein the titaniumhalide is reduced by passing the titanium halide through titaniumparticles at a temperature not less than 700° C.
 16. A method accordingto claim 7 wherein the substrate is metallic, and wherein, prior toproviding the flow of the volatile halide of titanium, the substrate isheated to a temperature above about 600° C. and is subjected to a flowof a gaseous mixture of boron chloride and hydrogen to produceboronization of the outer surface of the substrate.